Bi-level digit line architecture for high density drams

ABSTRACT

There is a bi-level bit line architecture. Specifically, there is a DRAM memory cell and cell array that allows for six square feature area (6F 2 ) cell sizes and avoids the signal to noise problems. Uniquely, the digit lines are designed to lie on top of each other like a double decker overpass road. Additionally, this design allows each digit line to be routed on both conductor layers, for equal lengths of the array, to provide balanced impedance. Now noise will appear as a common mode noise on both lines, and not as differential mode noise that would degrade the sensing operation. Furthermore, digit to digit coupling is nearly eliminated because of the twist design.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/211,662,filed Dec. 15, 1998, now U.S. Pat. No. 6,084,307 which is a continuationof application Ser. No. 08/950,471, filed Oct. 15, 1997, now U.S. Pat.No. 5,864,181, issued Jan. 26, 1999, which is a continuation ofapplication Ser. No. 08/442,264, filed May 15, 1995, abandoned, which isa continuation of application Ser. No. 08/123,027, filed Sep. 15, 1993,abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to integrated circuits (ICs).Particularly, there is a RAM device where digit and digit bar, definedas a pair, are laid out vertically (in the z-axis) to each other,whereas the pairs of digit lines are laid out to be parallel (in the xor y axis) to each other. Additionally, the vertically aligned digitline pairs allow usage of memory cells having a six square feature area(6F²) or less, where F is defined as the minimum realizablephotolithographic process dimension feature size.

2. State of the Art

Dynamic random access memory (DRAM) production in the early daysresulted in large chips. Manufacturing of these chips, at first, was notconcerned with shrinking every part down to its smallest size. At thistime the open memory array was the standard design: true digit lines onone side and complement digit (also known as digit bar or digit*) lineson the opposite side, with sense amps in the middle. However, once theDRAMS reached the 256K memory density, shrinking of all features becameimportant.

However, to push to even higher densities, like a one Megabit density,the open architecture proved to be inadequate because of the poorersignal to noise problem. As a result, the folded bit line architecturewas developed. Yet, to use this architecture, the original memory cellfrom the open architecture could not be used. Thus, new cells weredesigned. There resulted a memory cell with a minimum size of eightsquare feature area (8F²). The folded architecture eliminated the signalto noise problems. Thus, further shrinkage of the other components onthe DRAM resulted in an overall smaller DRAM package.

Problem

For some time now, there have been many ways developed to shrink the diesize. However, a new shrinkage barrier has been reached as designsapproach densities of 16 and 64 Meg chips. Every aspect of the die nowhas to be designed with minimal size. Thus, it is now necessary toshrink the previously acceptable eight square feature area (8F²) cells.Cell sizes of six square feature area (6F²) to four square feature area(4F²) are now needed. As a result, customers now need memory cells ofsix square feature area (6F²) or smaller that will also avoid theprevious signal to noise ratio problems.

Note, the above described problem, as well as other problems, is solvedthrough the subject invention and will become more apparent, to oneskilled in the art, from the detailed description of the subjectinvention.

BRIEF SUMMARY OF THE INVENTION

One skilled in the art will appreciate the advantage of the bi-level bitline architecture. Specifically, there is a DRAM memory cell and cellarray that allows for six square feature area (6F²) cell sizes andavoids the signal to noise problems. Uniquely, the digit lines aredesigned to lay on top of each other like a double decker overpass road.Additionally, this design allows routing of digit lines on bothconductor layers, for equal lengths of the array, to provide balancedimpedance. Now noise will appear as a common mode noise on both lines,and not as differential mode noise that would degrade the sensingoperation. Furthermore, digit to digit coupling is nearly eliminatedbecause of the twist design.

To achieve the digit line switching, several modes of vertical twistingwere developed. For a given section of the array, the twists arealternated between adjacent digit line pairs such that the overall twistresembles the traditional folded digit line twist. This twisting of thelines ensures that the signal to noise ratio of the bi-level digit linearchitecture can be as good as or may be even better than the foldeddigit line.

Other features and advantages of the present invention may become moreclear from the following detailed description of the invention, taken inconjunction with the accompanying drawings and claims, or may be learnedby the practice of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is an illustration of one embodiment of the invention.

FIG. 2 is an illustration of one embodiment of the invention.

FIG. 3 is an illustration of one embodiment of the invention.

FIG. 4 is an illustration of one embodiment of the invention.

FIG. 5 is an oblique view of a portion of the memory array showing thelocation of the twists, sense amps, row decodes, and isolation devices.

FIG. 6 is a representation of the layout of the present invention.

FIG. 7 is a suggested layout for a portion of a DRAM memory array havingtwisted double-layer digit line pairs.

FIG. 8 is an alternative suggested layout for a portion of a DRAM memoryarray having twisted double-layer digit line pairs.

It is noted that the drawings of the invention are not to scale. Thedrawings are merely schematic representations, and not intended toportray specific parameters of the invention. The drawings are intendedto depict only typical embodiments of the invention, and are thereforenot to be considered limiting of its scope. The invention will bedescribed with additional specificity and detail through the use of theaccompanying drawings, specification, and claims. Additionally, likenumbering in the drawings represents like elements within and betweendrawings.

DETAILED DESCRIPTION OF THE INVENTION

Incorporated Material

The following U.S. patents are herein incorporated by reference forpertinent and supporting information:

U.S. Pat. No. 5,208,180, is a method of forming a capacitor.

U.S. Pat. No. 5,206,183, is a method of forming a bit line over acapacitor array of memory cells.

U.S. Pat. No. 5,138,412, is a dynamic RAM having an improved largecapacitance.

U.S. Pat. No. 4,742,018, is a process for producing memory cells havingstacked capacitors.

U.S. Pat. No. 4,970,564, is a semiconductor memory device having stackedcapacitor cells.

U.S. Pat. No. 4,536,947, is a CMOS process for fabricating integratedcircuits, particularly dynamic memory cells with storage capacitors.

General Embodiment

One skilled in the DRAM semiconductor memory cell history and art willeasily understand the operation of this Bi-Level Digit line design usingan open architecture memory cell of six square feature area (6F²) orsmaller feature size and switching of the digit line levels to eliminatethe signal to noise ratio problems of the past.

This invention provides a new architecture for a dynamic random accessmemory (DRAM). The memory is characterized as having a plurality ofdigit line pairs, with each digit line pair consisting of both a truedigit line and a complement digit line. Both digit lines of each digitline pair are electrically insulated from one another by a dielectriclayer and vertically aligned along a major portion of their lengths. Atone or more positions along their lengths, their positions with respectto one another are reversed. In other words, if the true digit line isinitially on top during a first portion of the full length of the pair,the complement digit line is on the bottom and makes contact to aplurality of cells by means of digit line contacts. Using one of thetwisting techniques depicted in FIGS. 1 to 4, the complement digit lineis brought to the uppermost position while the true digit line isbrought to the lowermost position.

Further illustrated in FIG. 7 are isolation gates/lines 83 which keepthe two adjacent memory cells from biasing each other. Such isolationgates/lines 83 are grounded and are formed of polysilicon and/or othermaterial, such as an insulator material. By having such isolationgates/lines 83 grounded, the adjacent memory cells may be moreeffectively prevented from biasing each other during operation whilehaving higher potentials applied thereto.

Referring to drawing FIG. 8, an alternative embodiment of the digit lineswitching, using vertical twisting, is illustrated. As illustrated, withrespect to digit line pair DPO including upper digit line D* and lowerdigit D, both metal digit lines, the right-hand portion of upper digitline D* is connected by means of right standard contact 94 topolysilicon area 90 and connected by means of left standard contact 94from the polysilicon area 90 to the left-hand portion of upper digitline D* while lower digit line D is insulated from the polysilicon area90 passing thereabove and thereover. When considering digit line pairDP1, upper digit line D* extends to cross or to overlie a portion ofdigit line D of digit line pair DPO, extends to bit contact 96, andextends over left standard contact 94, being insulated therefrom at theupper level of the digit line pair DP1 of the array while right-handportion of lower digit line D of the digit line pair DP1 extends toright standard contact 94, in turn, connected to N+ active area 92, inturn, being connected by left standard contact 94 to the left-handportion of the lower digit line D of the digit line pair DP1. In eachinstance, when considering the right-hand standard contact 94, prior tosuch contact, both digit lines D* and D are located vertically withrespect to each other prior thereto in the array and when consideringthe left standard contact 94, from thereon both digit lines D* and D arelocated vertically with respect to each other from thereon in the array.Furthermore, the pattern for the arrangement of the digit lines isrepeated with respect to digit line pairs DP2, DP3, DP4, and DP5 asdescribed hereinbefore. In this manner, the noise is balanced throughthe use of vertical twists of the digit line pairs and the use ofpolysilicon areas and active N+ areas of the array. Additionallyillustrated and described herein are grounded gate isolation areas 83,word lines 82, and bit line contacts 81.

FIG. 1 illustrates one embodiment of the vertical three level downwardtwist design to achieve equal bit line lengths on the top and bottom ofthe design. As illustrated, on the left side of the FIG. 1, D (digit)line 10 (also referred to as “plane 10”) is located directly above D* (Dbar) line 12 (also referred to as “plane 12”). It is noted that D line10 drops down to a first plane 14, then to a third plane 16, and isrouted around the D* line 12 and then elevated back up to the firstplane 14. At the second level, D line 10 has achieved a twist in thevertical direction or Z-axis. A similar vertical rotation occurs for D*line 12, except it drops down only one level to plane 18, and proceedsaround the third plane 16 location and then elevates to a same secondplane 12, and then to plane 22, where it will remain until the nexttwist is encountered.

It is noted that planes 10 and 22 are on the same level, as well asplanes 12 and 14, and planes 16 and 18, respectively. It also is notedthat all of the twisting is relatively in a z direction and that at onlytwo points does the twisting require additional X-Y plane real-estate,that being on level 18 and 16.

Review of FIG. 2 shows almost an identical twist. However, there arefour levels in this twist. Level 4, or plane 19, is located below level3 and plane 16. Level 4 could be any material, like substrate implant,polysilicon, metal 1, etc., the key factor being that planes 19 and 16don't create a transistor. A variation of this design is to have plane19 arranged like plane 18 in FIG. 1 to avoid a transistor if thematerial would create such.

Review of FIG. 3 illustrates a three level twist up architecture. Asillustrated, the two digit lines are on the bottom planes 12, 14, 16 and18. Whereas the twisting takes place on the upper planes 10 and 22.Again, all the planes are in a vertical orientation to one another.However, planes 10 and 22 do project out into the X-Y planes toaccomplish the twist.

Review of FIG. 4 illustrates a four level downward twist. Digit line (D)30 is moved down one level via planes 32 and 34, while digit bar (D*) istwisted upward via planes 42 to 40. It is noted that plane or line 42 isthe only plane to extend in the X-Y plane, and, in fact, it extends intothe vertical plane of an adjoining pair of digit lines. To accommodatethis extension, the bottom line 48 of D* is moved to a fourth lowerlevel or plane 50, and then brought back up to line 52, while digit line46 has no need to be repositioned since it is elevated above the plane42.

Review of FIG. 5 illustrates a DRAM and an oblique view of two sectionsof the array utilizing the bi-level twist architecture. It is notedthat, although there are two digit line pairs illustrated, they are infact vertically oriented, one lying on top of the other. Additionally,the X 68 marks illustrate where the twisting takes place. It is notedthat each line in each pair will spend 50% of the length located on thebottom of the vertical architecture. For example, upper line 60 switchesto lower line 66 and lower line 64 moves up to the upper line 62. Ofcourse, the appropriate memory cells will be located near the correctbit line sections to receive the information stored in the cells andfeed that into the sense amps 70. An advantage with this architecture isthat the row decoders 72, attached to the row lines 73, can bepositioned on one side of the array. Additionally, the isolation lines74 are also symmetrical per array and thus can share a common groundingnode 76 located between the two arrays illustrated.

Attending to FIG. 6, there is an overview of a DRAM exhibiting eightmemory cells 84 and the appropriate lines as illustrated. In particular,there is active area 80 running the length of bit lines 86 (though oneline is shown, both the D and D* lines are vertically oriented). Wordlines 82 will turn on the transistor to access the cells. Bit linecontacts 81 will dump the cell charge onto the lower of the digit lines.Isolation gates/lines 83 keep the two adjacent memory cells from biasingeach other.

Referring now to FIG. 7, a layout portion of a DRAM array havingdouble-layer twisted digit lines is depicted. Six digit line pairs (DP0,DP1, DP2, DP3, DP4 and DP5) are shown in this abbreviated layout. Itwill be noted that in the depicted portion of the array, only digit linepairs DP0, DP2 and DP4 undergo a twist. Digit line pairs DP1, DP3 andDP5 are untwisted in this portion of the array. The alternating twistpattern not only provides for efficient reduction of capacitive couplingbetween adjacent digit line pairs, but it also provides room for thetwisting operation. It will be noted that portions of first conductivestrip S1 and second conductive strip S2 are vertically aligned withportions of adjacent digit line pairs. This is possible because firstand second conductive strips S1 and S2 are not on a level with either ofthe adjacent double-layer digit lines. The memory cell layout to theright and left of the twist region 71 is similar to that depicted inFIG. 6. Vertical contact vias are represented by the squares marked withan “X”. The interconnect pattern is similar to that depicted in FIG. 1.In FIG. 1, Level 2, the digit lines located on planes 12 and 14 would beused to interconnect the corresponding pairs of adjacent contact vias.For example, for digit line pair DP2, the digit line located on plane 14would interconnect contact vias CV1 and CV2, while the digit linelocated on plane 12 would interconnect contact via CV3 and CV4.

Remarks about the Invention

It is noted that the signal to noise ratios are kept acceptably low. Thevertical arrangement and the crossing digit lines allow for equal topand bottom orientation and access to the appropriate memory cells.Additionally, the adjoining digit pair of lines is also switchedappropriately to diminish signal to noise problems.

It is further noted that this array arrangement allows for the smallercell sizes, for example, cells possible from the older open bit linearchitecture or any new six square feature area (6F²) or smaller cellsize, thus allowing smaller arrays using six square feature area (6F²)to four square feature area (4F²) cell sizes.

Still a further advantage is the overall arrangement of the cells, bitlines, word lines, and isolation lines. All devices and lines are laidout to be exactly straight. There is no routing around the cells to openthe gates like with the eight square feature area (8F²) designs of thefolded array structures.

Additionally, there is one sense amp (S-amp) located on one end of thedigit and digit bar lines in an alternating pattern of the S-amp.

It is also noted that the twisting locations in the array are at quartermarks, either the first and third quarter, or at the halfway mark in thearray. This allows for different digit line pair arrangements to belocated next to each other.

Variations in the Invention

There are several obvious variations to the broad invention and thuscome within the scope of the present invention. Uniquely, this inventionmay work with any positioning of the memory cells. Specifically, thecells may be located between, along side, on top, or underneath the bitlines, thus accommodating for trench, stacked, or elevated designs. Oneskilled in the art would have little trouble using the vertical bi-levelbit line arrangement with these other DRAM designs.

Additionally, any layering can be used for the bi-level digit lines.Specifically, the bottom layer could be an implant in the substrate, orpoly on top of the substrate, or any of the metals over the poly. It alldepends on how high the chip design is stacked and where the memorycells are located.

Similarly, the twisting of the vertical digit lines can be locatedanywhere in the array, like over {fraction (1/12)} of the line. The onlyrequirement is that half of the length of each digit line is located ontop and half on the bottom of the vertical arrangement, although it isnoted that any increase in the number of twists will increase the sizeof the array.

While the invention has been taught with specific reference to theseembodiments, someone skilled in the art will recognize that changes canbe made in form and detail without departing from the spirit and thescope of the invention. The described embodiments are to be consideredin all respects only as illustrative and not restrictive. The scope ofthe invention is, therefore, indicated by the appended claims ratherthan by the foregoing description. All changes which come within themeaning and range of equivalency of the claims are to be embraced withintheir scope.

What is claimed is:
 1. A memory device comprising: a multi-level digitline pair located on a portion of a surface of a semiconductor die in aplane, the multi-level digit line pair having a first digit line havinga width and having a second digit line having a width, the digit linepair having a vertical twist therein having the first digit line locatedbelow the second digit line on one horizontal side of the vertical twistand located above the second digit line as an upper digit line on anopposite horizontal side of the twist, the first digit line and thesecond digit line of the multi-level digit line pair being verticallyoffset and vertically aligned within the width of each digit line of themultilevel digit line pair along the length thereof on the onehorizontal side of the vertical twist and on the opposite horizontalside of the vertical twist, the multi-level digit line pair located inthe plane on the one horizontal side of the vertical twist and locatedin the plane on the opposite horizontal side of the vertical twist. 2.The memory device of claim 1, further comprising: a plurality ofselectively addressable memory cells coupled to the multi-level digitline pair such that the plurality of addressable memory cells is coupledto the first digit line and the second digit line when the first digitline and the second digit line are located below another digit line. 3.The memory device of claim 2, wherein the first digit line and thesecond digit line have an approximately equal number of selectivelyaddressable memory cells coupled thereto.
 4. The memory device of claim1, wherein the first digit line and the second digit line are fabricatedas a first digit line and a second digit line of one of a metal layerand a polysilicon layer.
 5. The memory device of claim 1, wherein thevertical twist comprises a conductive contact vertically extendingthrough the semiconductor die to electrically connect portions of thefirst digit line and the second digit line.
 6. The memory device ofclaim 5, wherein the conductive contact comprises one of a polysiliconplug and a metal plug.
 7. The memory device of claim 1, furthercomprising at least two or more vertical twists along a length of themulti-level digit line pair.
 8. An integrated circuit memory devicecomprising: an integrated circuit die having multiple vertically offsetconductive levels; a multi-level digit line pair fabricated in a planeextending along a portion of a surface of the integrated circuit die,the integrated circuit die having first and second electrically isolateddigit lines, the first digit line having a width and the second digitline having a width; each digit line of the first digit line and seconddigit line comprising first and second sections located in differentones of the multiple conductive levels, the first digit line and thesecond digit line being vertically offset in the plane, verticallyaligned within the width thereof, and electrically connected; and thefirst digit line and the second digit line located in the plane suchthat the first section of the first digit line is vertically locatedabove the first section of the second digit line and the second sectionof the first digit line is vertically located below the second sectionof the second digit line.
 9. The integrated circuit memory device ofclaim 8, further comprising a conductive line connected between avertically traversing electrical path and a section of a digit line ofthe first digit line and the second digit line, the conductive linelocated in a different conductive level than the first digit line andthe second digit line.
 10. The integrated circuit memory device of claim8, further comprising a plurality of selectively addressable memorycells coupled to the first digit line and the second digit line suchthat the plurality of addressable memory cells is coupled to the firstdigit line and the second digit line when the first and second digitlines are located in a lower conductive level.
 11. The integratedcircuit memory device of claim 10, wherein the first digit line and thesecond digit line have an approximately equal number of selectivelyaddressable memory cells coupled thereto.
 12. The integrated circuitmemory device of claim 10, further comprising isolation regionscomprising isolation transistors located between the plurality ofselectively addressable memory cells, each isolation transistor having agate connected to a bias potential.
 13. The integrated circuit memorydevice of claim 12, wherein the bias potential is negative.
 14. Theintegrated circuit memory device of claim 8, further comprising an arrayof memory cells arranged in a cross point architecture.
 15. Theintegrated circuit memory device of claim 10, wherein the first digitline and second digit line have an approximately equal number ofselectively addressable memory cells coupled thereto.
 16. The integratedcircuit memory device of claim 8, wherein the first digit line and thesecond digit line are fabricated as one of a first metal layer, a secondmetal layer, and a polysilicon layer.
 17. The integrated circuit memorydevice of claim 9, wherein the vertically traversing electrical pathcomprises one of a metal plug and a polysilicon plug verticallyextending through the integrated circuit die.
 18. A memory devicecomprising: a multi-level digit line pair fabricated in a planeextending along a portion of a surface of a semiconductor die, themulti-level digit line pair having a first digit line having a width anda second digit line having a width, the digit line pair having avertical twist therein such that the first digit line is located belowthe second digit line on one horizontal side of the vertical twist andlocated above the second digit line as an upper digit line on anopposite horizontal side of the twist, the first digit line and thesecond digit line of the multi-level digit line pair being verticallyoffset and vertically aligned within the width of each other along thelength thereof on the one horizontal side of the vertical twist and onthe opposite horizontal side of the vertical twist, the multi-leveldigit line pair located in the plane on the one horizontal side of thevertical twist and located in the plane on the opposite horizontal sideof the vertical twist; and a plurality of addressable memory cellscoupled to the multi-level digit line pair such that the plurality ofaddressable memory cells is coupled to the first digit line and thesecond digit line when the first digit line and the second digit lineare located below another digit line of the multi-level digit line pair.19. The memory device of claim 18, further comprising isolationtransistors fabricated between adjacent ones of the plurality ofaddressable memory cells, each isolation transistor having a gateconnected to a bias potential therefor.